It said the claim of an aerial attack, which might have caused the descent of the plane by direct damage or by harassment, was capable of being proved or disproved.

The report said that given the NSA's worldwide monitoring activities at that time, "it is highly likely" that the radio traffic on 18-19 September 1961 was recorded by the NSA and possibly also by the CIA.

The report said: "Authenticated recordings of any such cockpit narrative or radio messages, if located, would furnish potentially conclusive evidence of what happened to the DC6."

The Commission said it had made Freedom of Information Act requests to the NSA, which were rejected on national security grounds - but that an appeal had been lodged.

The report concluded that Mr Hammarskjold's death was "an event of global significance which deserves the attention both of history and of justice".